SNOWPACK IS A CRUCIAL WATER RESOURCE NATIONALLY AND GLOBALLY BUT IS POORLY MEASURED. SNOWPACK ACCUMULATION IS OFTEN GREATEST IN ALPINE AREAS WHERE WIND SCOUR DRIFT AND RUGGED TOPOGRAPHY CREATE DRAMATIC SPATIAL VARIATIONS IN SNOW PROPERTIES LEADING TO A HIGHLY DIFFICULT ESTIMATION PROBLEM. IN ADDITION THE PRESENCE OF VEGETATION DRAMATICALLY INCREASES THE COMPLEXITY OF SNOW ESTIMATION GLOBALLY. GIVEN THESE PHYSIOGRAPHIC COMPLEXITIES NO SINGLE OBSERVATIONAL TECHNOLOGY CAN FULLY OVERCOME THE CHALLENGES ASSOCIATED WITH SNOWPACK ESTIMATION. NOTWITHSTANDING RECENT ADVANCES IN SNOW MEASUREMENTS AFFORD NEW OPPORTUNITIES FOR DEVELOPING SPACEBORNE SNOWPACK RETRIEVALS. IN THIS REGARD WE PROPOSE A DETAILED CHARACTERIZATION OF THE SPATIO-TEMPORAL VARIABILITY IN SNOW PROPERTIES AND THEIR CONTROLS ON MULTI-SENSOR MEASUREMENTS OF SNOW WATER EQUIVALENT (SWE). THIS PROPOSAL IS INTENDED TO DOVETAIL WITH OTHER PROPOSALS FOCUSED ON RETRIEVAL ALGORITHM DEVELOPMENT (E.G. LED BY M. DURAND OHIO STATE UNIVERSITY). THE SPECIFIC GOAL OF THIS PROPOSAL IS TO DEVELOP THE ROBUST AND COMPREHENSIVE GROUND-BASED DATA SETS NEEDED TO TRANSFORM SNOW REMOTE SENSING. THE PROPOSAL SETS FORTH AN EXPERIMENT PLAN THAT WOULD BE LED BY THE PI AND WOULD EVOLVE IN COLLABORATION WITH OTHER PI S SELECTED UNDER THIS NRA. WITH REGARD TO SPATIAL VARIABILITY THE PROPOSED RESEARCH WILL DEVELOP THE SCIENCE TO DETERMINE OPTIMAL SAMPLING STRATEGIES OF SNOW PROPERTIES ACROSS INTENSIVE STUDY AREAS (ISA S) IN WHICH SNOW DEPTH DENSITY TEMPERATURE AND GRAIN SIZE WILL BE MONITORED USING FIELD PORTABLE SPECTRAL SNOWPACK PROFILERS SPECTRORADIOMETERS AND A SNOW PENETRATING RADAR. THESE MEASUREMENTS REPRESENT A SUITE OF NEW TECHNOLOGIES NEVER BEFORE COMBINED IN A SINGLE EXPERIMENT. WITH REGARD TO TEMPORAL VARIABILITY THE PROPOSED RESEARCH WILL MAKE THE FIRST CONTINUOUS OBSERVATIONS OF SNOWPACK STRATIGRAPHY AT A LOCAL-SCALE SITE WITH A CONTINUOUSLY OPERATING SPECTRAL SNOWPACK PROFILER ACTIVE AND PASSIVE MICROWAVE SENSORS AND ARRAYS OF ULTRA-SONIC SNOW DEPTH SENSORS; A SUITE OF MEASUREMENTS WHICH ARE UNPRECEDENTED WITH REGARD TO CONTINUOUS SAMPLING. THE PROJECT OBJECTIVES INCLUDE: 1) DETERMINE THE SENSITIVITY OF REMOTE SNOW MEASUREMENTS (I.E. RADAR PM LIDAR AND HYPERSPECTRAL) TO SPATIAL VARIABILITY OF SNOW DEPTH GRAIN SIZE AND OTHER SNOW PROPERTIES. 2) DETERMINE THE INFLUENCE OF TEMPORAL VARIABILITY IN SNOWPACK STRATIGRAPHY ON MODELS AND OBSERVATIONS OF SNOWPACK ELECTROMAGNETIC SIGNATURES (E.G. FROM RADAR PM LIDAR HYPERSPECTRAL AND MULTI-SPECTRAL SENSORS). THE DATA THAT WILL BE DEVELOPED UNDER THIS RESEARCH WILL SERVE BROADER NASA SNOW REMOTE SENSING GOALS BY PROVIDING THE GROUNDBASED GRAIN SIZE MEASUREMENTS FOR DEVELOPING SWE RETRIEVALS FROM MICROWAVE MEASUREMENTS FROM GROUND-BASED AIRBORNE AND FUTURE SPACEBORNE PLATFORMS. THE MEASUREMENTS WILL ALSO ESTABLISH THE NECESSARY ACCURACIES OF MODEL-BASED SNOW PROPERTY ESTIMATES TO FACILITATE REMOTE SWE RETRIEVALS. THE FIELD DATA SET IS EXPECTED TO EMPOWER THE DEVELOPMENT AND TESTING OF NEW MODELS AND OBSERVING SYSTEMS THAT INCLUDE AND GO BEYOND REMOTE SENSING CAPABILITIES IDENTIFIED BY THE INTERNATIONAL SNOW REMOTE SENSING WORKING GROUP SERVING THE BROADER COMMUNITY AND PROVIDING THE BASIS FOR COLLABORATIONS ACROSS THE SNOW COMMUNITY.